Many industrial operations utilize fluoride, often as hydrofluoric acid or as fluoride salts such as ammonium fluoride. For example, alumina and silica etching, cleaning, etc. and semiconductor production utilize large amounts of hydrofluoric acid, and other fluoride compounds. As a regulated element in water discharge, for reasons well known in the art, the wastewater generated needs to be processed for fluoride ion removal. Additionally, when using hydrofluoric acid (HF), semiconductor manufacturers often require an ultra-pure hydrofluoric acid.
The typical semiconductor manufacturer may produce an average of 10,000 gallons per day of mixed acidic fluoride waste. The production of such vast quantities of fluoride ion waste, however, presents significant disposal problems. Fluoride wastes are becoming subject to increasingly stringent environmental controls for treatment and disposal. Industry must therefore greatly reduce the fluoride content of waste solutions before the solutions may be introduced into the municipal water disposal system.
With the increased use of fluorine as a chamber cleaning gas, the volumes of waste are expected to increase. The state-of-the-art practices for fluoride treatment have focused on precipitation of fluoride as an insoluble calcium fluoride (CaF2) salt by treating the dilute streams with either direct calcium hydroxide addition (Ca(OH)2) or forming calcium hydroxide by addition of calcium chloride (CaCl2) and sodium hydroxide (NaOH).                The reaction proceeds as follows:2Ca(OH)2+2HF+H2SO4=CaF2+CaSO4·2H2O+2H2O        
As can be seen from the reaction, on a dry weight basis, the sludge would contain only 45% calcium fluoride (CaF2), the balance being relatively benign CaSO4.
This treatment scheme suffers from several drawbacks. Solubility of (Ca(OH)2) in water is approximately 1,600 ppm. Therefore, addition of Ca(OH)2 typically results in the injection of a slurry of Ca(OH)2. The slurry has 10–15 micron size particles of Ca(OH)2 that act as the seed for calcium fluoride (CaF2) precipitation. The sulfates present in the water also precipitate since calcium sulfate (CaSO4) is not very soluble. Also as CaSO4 has two molecules of water attached to the salt molecule, the precipitated solid is very sticky, and does not filter well in the filter press. As a result, filter press operations may need to be stopped prematurely due to pressure build-up, resulting in excessive amounts of moisture being left in the cake. A further limitation of these systems has been the level of fluoride ion concentration that they can achieve. Typically, at slightly alkaline pH, approximately 20 parts per million (ppm) of CaF2 is still soluble in the water. At excessive dosage levels of reactants, especially the calcium and alkalinity source, lower levels of 10–12 ppm of fluoride can be achieved. However, in some areas of the world, the discharge limits for fluoride are being lowered to sub 2-ppm levels. This level cannot be technically achieved by the precipitation mechanism due to the lower solubility limit of CaF2.
In addition, it is very difficult for the semiconductor manufacturer to recycle the fluoride after this process has been used, because caked calcium fluoride also contains high amounts of silicon as silica, which is difficult to separate from fluoride. As silica has adverse effects on many semiconductor manufacturing processes, its presence in the caked calcium fluoride negates its value as a raw material. Therefore, all of the fluoride from conventional fluoride caking systems is unusable by semiconductor manufacturers, because it is unavailable for recycle or recovery.
In addition to the above problems, the industry desires a way to isolate and treat the waste streams containing ammonium ions. Current state-of-the-art employs biological filters which require much maintenance and are not very efficient.
U.S. Pat. No. 5,876,685 discloses a method for the removal and purification of substantially all the fluoride ions contained in a solution containing greater than 10 parts per million (ppm) fluoride ion, a mixture of other anions, silicon in the form of a fluorosilic acid, silicic acid, silicates, or silicon tetrafluoride, and optionally also containing complex metal fluorides, to produce a hydrofluoric acid.